Unstable or oscillatory flow conditions within a turbocompressor, known as surge and stall, are detrimental to process machinery and to the overall process operation. The proximity of the compressor to these unfavorable conditions is detected by process monitoring apparatuses that interact with control algorithms which regulate compressor flow rates within a stable operating region, thus avoiding surge and stall.
Surge control is initiated by analog input signals emanating from various sources located throughout the compressor-process system. Although these signals are many, the set used must consist of relevant data to initiate control-algorithm response (by recirculating or blowing off some of the process gas) to any disturbance before the process flow rate reaches a surge condition.
Prior art surge control can be divided into two categories: surge parameters which are invariant to inlet conditions, and those parameters which are not. Invariant parameters in the prior art consist of different combinations of reduced flow and pressure ratio; or combinations of volumetric flow divided by rotational speed, and polytropic head divided by rotational speed squared. The calculation of these parameters requires knowledge of at least the pressures at the suction and discharge of the turbocompressor, and a flow measurement (.DELTA.p.sub.o). One advantage of the present invention is that it is not limited to this combination of transmitter signals. Control strategies can be implemented using, for instance, a power measurement, suction pressure, and discharge pressure. Furthermore, the concept of this invention can be applied to the detection of fault and fallback strategies, which will keep the turbocompressors running under adverse circumstances.
Of the second category of parameters (those not invariant), some are based on the same pressure and flow measurements as the first category, while others utilize a power or rotational speed measurement as a replacement for flow or discharge pressure measurement. Thus, a control scheme can be applied even if the turbocompressor lacks a flow or discharge pressure measurement. The advantage of the present invention over this prior art is that it does not require that corrections be made for changing inlet conditions.
Thus, there is a need for a method of surge control that provides the flexibility of having a multiplicity of control strategies, together with fault checking and fallback. There is also a need for a surge control system, invariant to inlet conditions, that can accommodate compressor-process systems which are not fully instrumented or have faulty transmitters. A typical turbocompressor performance map (FIG. 5) will depict a surge region (zone) and a stable operating region that are separated by a sharp interface referred to as the Surge Limit Line. Also shown on this map is a Surge Control Line, and the distance between this line and the Surge Limit Line is a safety margin. If the operating point crosses the Surge Control Line, into the safety margin, the antisurge controller calculates a finite error; this error is used in the PI loop. The output of the loop is used to activate an electromechanical sequence in which gas is recycled or blown off to reestablish and maintain a safe flow rate. Should this safety margin be excessive, the frequency and duration of flow recycling will increase, resulting in a reduction of energy efficiency of the compression process. Conversely, should the margin be too brief, the prospect of inadequate protection is amplified.
It is, therefore, obvious that considerable economic advantages can be derived from a narrow margin of safety that incorporates enhanced surge protection with a resultant lessening of process upset. Additional spin-off benefits would: better ensure efficient operation; extend the intervals between scheduled shutdowns; and increase annual monetary savings.
For the foregoing reasons, there is a need to easily and accurately calculate (using invariant coordinate systems) at what point instability occurs under all inlet conditions.